As a high density optical information recording medium, there is a proposal of a multilayer optical information recording medium having a plurality of signal recording layers in the thickness direction such as a DVD provided with two layers for reproduction on one side. A DVD provided with two layers for reproduction on one side, for example, has a structure with two substrates, one having an information recording surface on which a translucent reflecting layer of gold or silicon, etc., is formed and the other having an information recording surface on which a conventional reflecting layer made of aluminum, etc., is formed and the two substrates are pasted together in such a way that these information recording surfaces come inside facing each other. To further improve surface recording density per one layer, there is a proposal of a high density optical information recording medium having a violet laser light source (with a wavelength of approximately 400 nm), a lens of high NA and a 0.1 mm thick low-profile transparent cover layer on the recording/reproducing side. This high density optical information recording medium has a structure with signal guide grooves or pits formed on the surface of a thick signal substrate, a rewritable recording multilayer film formed thereon and a transparent cover layer further formed thereon.
For this low-profile transparent cover layer type high density optical information recording medium, there can also be one with two signal recording surfaces. One example of the method of manufacturing it will be shown below.
(1) On a thick substrate on which a rewritable recording multilayer film with signal guide grooves or pits formed on the surface is formed, a separation layer is further formed using UV cure resin and a second layer with signal guide grooves or pits is formed on the surface of the separation layer.
(2) A rewritable translucent recording multilayer film is formed on the second layer with signal guide grooves or pits.
(3) A low-profile recording/reproducing side transparent cover layer of 0.1 mm in thickness is formed.
An example of specific methods of manufacturing this is the one described in Japanese Patent Laid-Open No. 2002-260307. The entire disclosure of Japanese Patent Laid-Open No. 2002-260307 is incorporated herein by reference in its entirety. That is, according to the method of Japanese Patent Laid-Open No. 2002-260307, using a plastic stamper for the step in (1) above, UV cure resin is applied to signal guide grooves or pits on the stamper, which are thereby cured, then this is pasted to the substrate on which the first recording multilayer film is formed using other UV cure resin having a different nature and the stamper is peeled off after the cure. Using such a method makes it possible to create a multilayer optical information recording medium using a rigid thick substrate as the base and placing one or a plurality of signal recording layers on top of the base through a separation layer.
However, the separation layer lying between the signal recording layers of the multilayer optical information recording medium must be uniform. When the separation layer is uniform, influences of reflected light from the other layer during recording or reproduction become constant in the signal recording layers before and after the separation layer. When the reflected light from the other signal recording layer fluctuates, this becomes a disturbance component of the reproduced signal and deteriorates S/N. On the contrary, when the separation layer is uniform, disturbance from the other signal recording layer is constant, which stabilizes recording or reproduction and improves the quality of the reproduced signal. In practice, however, variations in the thickness of the separation layer are produced when on a thick substrate on which a rewritable recording multilayer film with a surface provided with guide grooves or pits is formed, a stamper is used to transfer the separation layer between the substrate and stamper and the signal guide grooves or pits from the stamper onto the surface.